The present invention relates to control of the gap between rolls of a rolling mill.
Rolling mills usually have to be controlled for keeping the thickness of the rolled strip constant, i.e. within specified tolerances. The control involves preferably a feedback loop, in which the gap between the working rolls is ascertained, e.g. by a contactless feeler, and the measured value is used as control input to be compared with a reference, and the error signal controls the roll gap. Devices of this type are shown, for example, in U.S. Letters Pat. No. 3,662,576; see also U.S. Pat. No. 3,817,068 and U.S. Pat. No. 3,850,015.
These devices for gap width pickup and detection are quite suitable to provide the primary input for a feedback system. It has to be observed, however, that there is not a 1:1 relation between rolling gap and thickness of the rolled stock. The latter may vary, for example, on account of changes in hardness or the blank that is being rolled varies in thickness. Also, the frame may change, the rolls may blend or flatten and there may be other causes changing the thickness of the rolled stock, even though the gap has remained within the specified tolerances. Swelling of the stock may compound the problem.
In order to offset and avoid these other interferences with the process of rolling stock at constant thickness, other and/or additional controllers are provided. For example, it is known to provide controllers for roll pressure; or for the spindle force or forth roll position. These various controllers, often used in the alternative, are only a partial solution to the whole problem, because they frequently do not respond to the actual source for the impending error, because the respective input transducers are not suited for that purpose or they are at the wrong location or both. Clearly, the closer the tolerances, the more compounded is the problem.
One could, for example, provide for a kind of master control in that the strip thickness itself is the controlled variable, and that thickness itself is ascertained by an appropriate transducer which, through feedback, controls the rolling gap to obtain constant thickness. However, for this particular kind of control it is inherent that the distance -- (in the direction of rolling) between thickness measuring transducer and rolling gap, divided by rolling speed defines a period of time in which control cannot possibly be made effective. Also, stability requires relative slow response so that errors are eliminated quite slowly.